As is known, numerous road vehicle, in particular car, manufacturers are currently experimenting hybrid-propulsion vehicles, some of which have recently been marketed. Though “hybrid propulsion” normally refers to the use of two or more different propulsion systems, the solutions hitherto proposed are normally a combination of an internal combustion engine (hereinafter referred to simply as a “combustion engine”) and an electric motor, to exploit the advantages and compensate the disadvantages of each.
More specifically, the main advantage of a combustion engine, and the reason why it is used almost exclusively in road vehicles, is the fuel distance obtainable by virtue of the high energy concentration of the fuel employed (substantially hydrocarbons). On the other hand, a combustion engine produces harmful emissions, can only operate within a given speed range, thus requiring the use of auxiliary components (gearbox and clutch), and has a relatively high noise level.
Moreover, even within the operating range, the efficiency of a combustion engine is fairly low (about 20-30%), and variations in the steady-state operation of the engine considerably increase pollutant emissions.
An electric motor produces no “local” pollution, is quiet, highly efficient (about 80%), and capable of generating torque as of start-up. Application in road vehicles, however, has so far been impaired by the limited range of the electric batteries currently available, at least those of a size, weight and cost feasible for earth vehicles.
To eliminate or reduce the drawbacks associated with the use of one type of engine/motor, hybrid solutions of various configurations have been proposed enabling alternative or combined use of both propulsion systems.
More specifically, in a first known so-called “series hybrid” combination, the combustion engine serves solely to drive an electric current generator, which charges the batteries powering the electric motor: propulsion is therefore governed exclusively by the electric motor. This solution provides for a considerable reduction in consumption, by the combustion engine only being called upon to supply average power, and so being operating in optimum steady conditions.
In the “parallel hybrid” configuration, both the combustion engine and the electric motor are connected to the drive wheels by appropriate transmission mechanisms.
Known hybrid drive assemblies of the type briefly described above are complex, expensive, and bulky, which is why, to the Applicant's knowledge, they have never been marketed for compact, low-cost vehicles, such as scooters.
EP-A-0 908 343 discloses a hybrid drive assembly including an internal combustion engine having a drive shaft, an electric machine having a rotor connected to a primary shaft aligned to the engine drive shaft, a clutch interposed between the engine drive shaft and the electric machine rotor, and a transmission unit including a CVT interposed between the primary shaft and the wheel axle.
Besides being considerably complex and thus not adapted for use in small road vehicles such as scooters, the above-referenced known hybrid drive assembly has a drawback in that the electric machine is located upstream from the transmission unit, and in particular the CVT, so that, when the electric machines operates as a generator (regenerative brake), energy recovery efficiency is reduced.